This application claims priority to German Application No. 199 19 602.8 filed on Apr. 2, 1999 and International Application No. PCT/EP00/03444 filed on Apr. 15, 2000 and published in German as International Publication No. WO 00/67040 on Nov. 9, 2000, the entire contents of each are hereby incorporated by reference.
The invention relates to a current sensor working in accordance with the compensation principle with a primary winding through which the current to be measured flows, creating a magnetic field which can be compensated by compensation current flowing through a secondary winding, and by sensor means influenced by the magnetic field, which are down-streamed by a booster circuit, and which discharges the secondary winding connected in a series by a terminating resistor, with a pulse-duration modulated compensation signal.
Such a current sensor is known from DE-A-197 05 767. The known current sensor possesses a comparator, which is discharged at one comparator input with the measuring signal delivered by the sensor means, and on the other comparator input with a linear time base generated by a voltage generator. The comparator controls two reverse timing power amplifiers, between which the terminating resistor as well as the secondary winding is connected in a bridge circuit.
A current sensor is known from DE-OS-196 42 472, which uses a switchable booster for decreasing the power requirement for the compensation current, and for reducing the losses at an operation with excessive supply voltage, and which is controlled by a pulsing gating signal, which possesses a mark space ratio depending on the measured value.
EP-0 742 440 reveals a device for compensation current conversion, which uses a timed booster for the generation of the compensation current, which is connected in a series to a control amplifier. The pulse duration modulated timer is then carried from an oscillator at a firm frequency.
One of the disadvantages of the known current sensor is the fact that due to the frequency response of the booster circuit, only primary currents up to a certain upper frequency threshold can be measured. The current sensor can no longer follow the changes with frequencies of the primary current above the frequency threshold, so that the voltage does not drop via the terminating resistor, even though primary current is flowing through the primary winding.
Based on this state of technology, the task of the invention is to create a currency sensor, which can also be used at high primary current frequencies.
This task is solved by a current sensor in accordance with the disclosure herein.
The pulse-duration modulated compensation signal possesses timing frequencies above the converter frequency threshold. The frequency response of the booster circuit at the existing current to be measured ensures a measurable voltage drop via the terminating resistor. Furthermore, a low-pass filter arrangement for stabilizing the pulse-duration modulated compensation current is intended, which is down-streamed to the booster circuit, and contains inductances and capacitances, a filter frequency threshold below the resonance frequency of the converter, and below the timing frequency of the booster circuit, and which possesses excessive resonance, whereby the excessive resonance of the low-pass filter arrangement is damped by an RC element connected in parallel to the secondary winding and the terminating resistor.
The invention therefore uses the fact that the current sensor works as a current converter at high primary current frequencies. The energizing magnetic field is increasingly compensated at high primary current frequencies by the secondary currents flowing through the secondary winding caused by the reverse voltage. The secondary currents flowing through the secondary winding due to the converter behavior also result in a voltage drop at the terminating resistor. The voltage created at the terminating resistor due to the converter behavior gets stronger the higher the frequency of the primary current gets, in order to near the upper threshold value above the converter frequency threshold. In order for the current sensor relating to the invention to be usable independently of the primary current frequency, it must be ensured that no gap is formed in the frequency range between the converter behavior and the sensor behavior, in which the voltage at the terminating resistor drops substantially. This is achieved in particular by the timing frequencies of the pulse-duration modulated compensation signal being above the converter frequency threshold, and by the fact that the frequency response of the booster circuit, in particular that of the upper frequency threshold, also ensures a measurable voltage drop via the terminating resistor. Both measures combined ensure that a measurable voltage drop also occurs in a transition area between sensor behavior and converter behavior via the terminating resistor.
Additional beneficial designs and examples are subject to the attached claims.